The ultimate aim of the union between Christ and the Church is righteous rule—a co-reigning with Christ over creation in the new heavens and new earth. This union is not merely for relational joy but for fulfilling God’s purposes in the world. Marriages that cultivate fruitful sexual love model this divine calling, bringing order, stability, rule and purpose to the household.

A Joy Too Vague for Glory

The mysteries of sexual relationships are as profound as they are beautiful. In Proverbs 30:18-19, Agur marvels at four wondrous things, including “the way of a man with a virgin,” acknowledging the depth of this mystery. While we cannot plumb its depths entirely, we can begin to appreciate the design and purpose behind sexual relations. Here are two foundational truths and five crucial aspects expressed in the sexual union of marriage.

Foundational Truths about Sexual Relations

1. Sexual Relations Reflect Christ and the Church

In Ephesians 5:22-32, Paul reveals that the institution of marriage was designed as a reflection of the union between Christ and His Church. Jonathan Edwards argues that the marriage between Christ and the church was God’s primary grand design in creation: “Thus the grand design of God in all his works and dispensations is to present to his Son a spouse in perfect purity, beauty and glory from amongst mankind.”

Human marriages, then, are dim reflections of the ‘grand design of God’, offering a glimpse into the divine love and union between Christ and His people. Marriage is a tiny window that looks over the Grand Canyon. It is a child attempting to play a Beethoven Symphony.

Paul shows that this connection extends even to the sexual relations in marriage by quoting Genesis 2:24: “A man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh. This mystery is profound, and I am saying that it refers to Christ and the church” (Ephesians 5:31-32).

The reference to the “one flesh” union connects sexual relations in marriage with the relationship between Christ and the Church, elevating the significance of sex within marriage. Augustine writes: ‘it is of Christ and the Church that this is most truly understood: “And the two shall be one flesh.”’

Christopher Ash in his excellent book Married for God focuses on marriage as ‘sex in the service of God’. There is validity to looking at marriage and sex through that lens, the lens of usefulness or service or function.

I think that Sex, Dating and Relationships articulates this better. The authors argue that the primary way we should look at sex and marriage is to be thought of as our human relationships being in the image of God. This comes from thinking of marriage as primarilya dim reflection of Christ and the Church. Marriage might be described as ‘sex in the image of God.’

2. Sexual Relations are the Concentrated Essence of Marriage

Sexual union embodies the concentrated essence of marriage, where every distinct aspect of the marriage relationship—one-flesh union, intimate knowledge, fruitful love, exclusive faithfulness, and overwhelming delight—is on vivid display. This is why sexual intercourse is a stand-in for marriage.

In ancient biblical times, sexual intercourse was often the act that signified and established a marital union (e.g., Genesis 24:67; Genesis 29). The physical act is not merely a component of marriage but a microcosm of it—a vivid expression of its core truths.

A married couple is meant to live out each day in mundane ways what they express to one another passionately in the sexual union. This truth sets marriage apart as a unique human relationship. The five unique aspects of marriage as a human relationship — one-flesh union, intimate knowledge, fruitful love, exclusive faithfulness, and overwhelming delight —are highlighted and underlined in bold in the sexual union.

Combining these two foundational ideas, a couple expresses a microcosm of their marriage in the sexual act, but they also preach to themselves and experience in a small way something of what the relationship between Christ and the church is like. It is both the concentrated essence of marriage and the shadowy image of a heavenly relationship. Sex is both a high and a low.

Five Aspects of Marriage Expressed in Sexual Union

1. One Flesh Union

Marriage is about the union of two into one; it is a “one flesh” union. The act of sexual intercourse graphically displays this unity physically.

This truth is even expressed in our modern language. Words used to describe sex such as coitus, sexual intercourse, copulation, and even the word mating, all come from the idea of union. The term “sexual intercourse”, for example, derives from Latin roots sexus which means to ‘divide’ or ‘cut’, and intercourse meaning “to run between.” This linguistic origin underscores how sexual intercourse bridges the division between a male and a female, creating unity.

Eve, taken from Adam’s body and brought back to him in marriage, reflects unity restored. Similarly, the Church is united with Christ through the indwelling of the Spirit, transforming believers into His likeness. This union is not only relational but transformative, as believers grow to reflect Christ’s character and values. The one flesh union of marriage mirrors this profound spiritual reality, highlighting the intimate connection and shared identity of those joined together.

In marriage, the physical expression of one flesh is meant to be lived out in all areas of life—in caring for one another, aligning goals, and growing together. As Paul notes in Ephesians 5, husbands nourish and cherish their wives as their own bodies, and wives submit to their husbands as the body submits to the head. Over time, this union fosters mutual understanding and conformity to one another’s image, mirroring the transformative union believers experience with Christ. This shared life and purpose embody the divine calling of marriage.

2. Intimate Knowledge

Marriage is a human relationship designed for intimate knowledge.

‘And the man and his wife were both naked and were not ashamed’ (Genesis 2:25). The nakedness and lack of shame of Adam and Eve stems initially from their innocence as they were without sin, but it also points to their immaturity. Mature humanity wears clothes (see Revelation 1:13, 7:9 and 19:8-9).

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